Rechargeable lithium ion batteries (LIBs) have been successfully developed over the last two to three decades and widely used to power today's portable electronic devices. The aim at long-term success in electric vehicles and grid-scale renewable energy storage raises great challenges with respect to high energy densities, long cycle lives, good safety and low-costs of the batteries. It is recognized that conventional lithium ion battery systems are approaching their theoretical energy density limits and their lifetime and charging rate must be improved.
Rechargeable batteries based on organic electrode materials are potential alternatives to conventional lithium-ion batteries due to their tunable properties, environmental friendliness, flexibility, good safety, sustainability and relative low cost. The organic molecules can be divided into n-type organic, p-type organic, and bipolar organic molecules, in which the neutral molecule can be either oxidized to the positively charged state or reduced to the negatively charged state.
Different from conventional inorganic electrode materials in LIBs, the battery performance of organic electrodes greatly depends on their molecular structures, which can be desirably tailored by synthesis.
Porphyrins have an aromatic 18 π conjugated system delocalized over 24 core atoms which satisfy Hückel's (4n+2) π-electron rule. Many porphyrins and metalloporphyrins exist naturally and have a wide range of application, e.g. in light harvesting, as a catalyst, or in solar cells. Porphyrins and their derivatives with highly conjugated π-systems are suitable for efficient electron transfer processes due to the minimal structural change of the molecules during the removal or uptake of an electron. 16 π-electron and 20 π-electron “antiaromatic” porphyrins are accessible by the oxidation or reduction of the 18 π-electron systems of aromatic porphyrins and in some cases can be successfully isolated and structurally determined.
However, so far no organic electrode materials had been available which are able to provide batteries having high power, high energy densities as well as very good cyclic stabilities.